In connection with accommodation and cooling of a conventional secondary battery, by way of example, Japanese Patent Laying-Open No. 2004-47262 discloses an assembled battery having a plurality of thin secondary batteries electrically connected and stacked. In the assembled battery, a plurality of radiating members are arranged to fill spaces between the plurality of secondary batteries (space between electrode terminals of neighboring thin batteries). In the assembled battery, a cooling medium is caused to flow in one direction to cool the plurality of secondary batteries.
An example of the thin secondary battery described above is a bipolar secondary battery. A bipolar secondary battery generally has a structure having a plurality of bipolar electrodes stacked one after another with an electrolyte interposed. Here, a bipolar electrode means an electrode having a collector member formed as a sheet, with positive electrode active material provided on one surface and negative electrode active material provided on the other surface, of the collector foil. The bipolar secondary battery may have various shapes, and as an example, it has a thin plate shape.
When the bipolar secondary battery is charged/discharged, a current flows in the direction of stacking of the bipolar electrodes. Consequently, heat builds up in the bipolar secondary battery, and the temperature of bipolar secondary battery increases. As a method of cooling the bipolar secondary battery, a cooling medium may be caused to flow in a direction along a plane orthogonal to the stacking direction of the bipolar electrodes.
When the bipolar secondary battery is cooled by this method, however, temperature difference generates in the cooling medium at the upstream side and downstream side of the cooling medium and, hence, temperature difference naturally generates also in the plane mentioned above. Specifically, the temperature of cooling medium is lower at the upstream side than at the downstream side. Japanese Patent Laying-Open No. 2004-47262 does not disclose any method of solving such a problem.